In a moncoque automotive body structure, various body panel members are stamp formed of thin steel plate, and are joined by spot welding or the like so as to form the final body structure. In such a body structure, it is important to ensure an adequate rigidity to the floor panel assembly. For this purpose, it was previously proposed to provide a lateral floor tunnel that extends laterally across the floor panel under the front seats in addition to the more conventional longitudinal floor tunnel (see Japanese patent laid open publication No. 2002-302071. The resulting upward projection in the floor panel having a cross-shape is effective in increasing the rigidity of the floor panel.
However, when a heavy fuel tank is mounted in the recess defined by the lateral floor tunnel, an additional reinforcement is required, and there was some difficulty in preventing such an additional reinforcement from reducing the capacity of the fuel tank and/or the leg rooms of the vehicle occupants.
When a shift lever is mounted on the longitudinal floor tunnel, an added rigidity is required to ensure a favorable feel to the vehicle operator who operates the shift lever, and the cross-shaped floor tunnel was not found to be entirely satisfactory. Therefore, it is necessary to reinforce the part of the longitudinal floor tunnel around the shift lever for the purpose of ensuring the required rigidity. Welding a steel plate having a suitable thickness in a part of the longitudinal floor tunnel provides a required rigidity, but the need for such as added manufacturing step is not desirable from economic view points.